Construction of a Boron Chain on a Single Metal by Dehydrocoupling of Borane Ligands

Abstract

Borane coordination, B–H borane bond activation, and borane catenation via metal-mediated dehydrocoupling to form electron-precise B–B bonds are reported. The reaction of trans-[M(IMes)2Cl4] (M = W, Mo) (IMes = 1,3-bis​(2,4,6-trimethylphenyl)​imidazolin-2-ylidene) with borates Li[BH3R] (R = Mes, Dur; Mes = 2,4,6-Me3C6H2 and Dur = 2,3,5,6-Me4C6H) afforded the complexes [M(IMes)​(η2-H2BR)2(η1-H2BR)] (M = W: R = Mes 1, R = Dur 3; M = Mo: R = Mes 2, R = Dur 4). Three borane ligands are coordinated in 1 – 4 to the group 6 metal atom via five (σ-B–H) bonds. Reaction of 1 with the phosphines PMe3 and PEt3, respectively, led to the elimination of one of the borane ligands and afforded the hydrido (σ-B–H)-boryl bis(σ-B–H)-borane complexes trans-[W(IMes)​(PR3)(η1-HBMes)​(η2-H2BMes)​(H)] (R = Me 5, R = Et 6), in which the metal atom inserted into one of the remaining σ-B–H bonds of the borane ligands. Reaction of 1 with an additional equivalent borane BH2Mes resulted in borane dehydrocoupling and formation of complex [W(IMes)​(η4-BH2Mes-BMes-BMes-BH2Mes)] 7, featuring a unique B4 chain as a ligand. Reaction of trans-[W(IMes)2Cl4] with NaBH4 also led to B–B coupling, and the metallaborane cluster [{W(IMes)​(BH4)}2(B5H9)] 9 was formed, in which two tungsten atoms bridge a B5 chain.